The object of the above-mentioned invention is a method for crystallizing a filler, especially calcium carbonate, in connection with a fiber web process, and an approach system for a fiber web machine. The invention especially relates to a method for continuous production of PCC used as a filler in the production of paper and cardboard in connection with a paper machine process.
Calcium carbonate is generally used in paper production both as a filler and a coating material, due to factors such as the high brightness of carbonate and its favorable price, among other things. Calcium carbonate may be produced by grinding either chalk, marble or lime stone, whereby it is referred to as ground calcium carbonate, commonly abbreviated as GCC (Ground Calcium Carbonate). Another way of producing calcium carbonate is a chemical method, in which, e.g., calcium ions present as a second component of calcium hydroxide and carbonate ions obtained when dissolving carbon dioxide in water are allowed to react, whereby the resulting calcium carbonate is precipitated from the solution as crystals, whose shape depends on the reaction conditions, among other things. The final product of this production method is referred to by the name PCC, which is an abbreviation of the words Precipitated Calcium Carbonate. This invention focuses on the production of PCC and its specific use as a filler in paper.
PCC production has traditionally been done separate from actual paper production. Until now, PCC has been produced either in its own separate plant located close to a paper mill from where the PCC slurry is conveyed by pumping trough pipelines to paper manufacture, or in a similar plant, from where the PCC slurry is transported by tank trucks to paper mills located at a distance. PCC produced by this method requires the use of retention aids in the paper production, so that PCC is able to adhere to the fibers, regardless of whether they are chemically or mechanically produced cellulose fibers. The above briefly described conventional way of producing PCC involves problems in addition to the already mentioned problem related to the use of retention aids. The transport of PCC by tank trucks to a paper mill from a chemical plant causes transport costs and requires use of dispersion agents and biocides. The use of these additives degrades the properties of PCC.
Construction of a separate PCC plant in connection with a factory is an expensive investment and requires many individuals working around-the-clock. A PCC plant also consumes a lot of fresh water and energy.
Thus, in order to reduce the production cost of paper, many different proposals for producing PCC directly at a paper mill have been made recently, thereby eliminating at least the transport costs of PCC from the papermaking cost structure. It has also been noticed that the production of PCC in the presence of fiber suspension leads to improved adhesion of the PCC crystals to fibers, and therefore reduces at least the need for retention aids, and sometimes their use can even be entirely avoided. The following description discusses several patent documents addressing the production of PCC in connection with paper manufacture.
WO-A1-0107365 discusses the conventional PCC production method. Generally speaking, this method assumes the formation of a suspension containing calcium ions, in which calcium exists both in dissolved ion form and as an insoluble solid. This suspension is supplied to a positive pressure reactor, into which gaseous carbon dioxide is introduced, and whose content is mixed continuously. Thus, the reaction of carbon dioxide and water creates carbonate ions, whose further reaction with calcium ions creates calcium carbonate. The process is run on a batch-type basis according to the pH value. In the initial stage, i.e., before feeding carbon dioxide, the pH value of the suspension present in the reactor is roughly above 12. Feeding carbon dioxide into the reactor is continued until the pH drops to the 6.5-7.5 level, whereupon the PCC suspension may be pumped through a screen to a storage tank for the papermaking process.
Calcium ions originating from the process described generally above may be either calcium oxide or calcium hydroxide. The document explains how the burned lime (CaO) is conveyed to the PCC plant by train, truck or pneumatic pipeline from a local silo. The burnt lime is slaked by mill water in a slaker in order for calcium hydroxide to form. If required, steam may be added in order to ensure a proper reaction temperature. The lime milk obtained from the slaker is pumped through a screen, in which the large-sized particles are removed, and into the aforesaid reactor. Besides the CO2 introduced as a pure liquid into the process, any suitable flue gas purified in an applicable manner may be used as a source of carbon dioxide being fed to the reactor. WO-A1-9935333, WO-A1-9945202 and WO-A1-0047817 discuss the protection of either ground calcium carbonate or PCC with acid paper production in mind. These documents assume that calcium carbonate is relatively stable under alkaline conditions, and that the low pH (roughly 8 and below) tends to split the calcium carbonate into calcium ions and carbonate ions, so that the suspension releases carbon dioxide, whose formation is noticeable as stock foaming. According to the document, it was noticed that by using carbon dioxide, whether in gaseous form or dissolved in water, calcium carbonate may be stabilized such that the pH of the paper production may be reduced to within a range of 6.5-7.5 without the risk of calcium carbonate decomposing.
U.S. Pat. No. 5,223,090 discloses the precipitation of PCC in fibers occurring at a paper mill, utilizing the hydrophilic properties of virgin pulp fibers by first mixing virgin pulp with lime milk and bringing the suspension with a water content of 40-95% into contact with a gaseous or liquid reagent (carbon dioxide), under vigorous mixing, in a pressurized tank in order to precipitate the filler particles in the fibers and especially their lumen. As a mixer alternative, a pressurized pulp grinder is disclosed.
U.S. Pat. No. 5,262,006 discusses a papermaking process, where in the recycled-fiber- or reject-containing suspension there is calcium sulfate or gypsum, which is used as raw material in the paper coating. The aim of this document is to convert at least a part of the recycled plaster to calcium carbonate. This is done by supplying alkali metals or ammonium carbonate to the suspension. Moreover, carbonate or hydrocarbonate ions are supplied to the paper making stock in order to form PCC, which causes the precipitation of calcium carbonate in the fibers. The carbonate ions, in turn, may be produced in a known manner from lime milk and carbon dioxide by first supplying lime milk and then carbon dioxide to the suspension. The relevant fiber suspension is either used as such or mixed with other cellulose components for paper production.
U.S. Pat. No. 5,558,782, U.S. Pat. No. 5,733,461, U.S. Pat. No. 5,830,364 and EP-A1-0658606 describe a process, in which alkaline earth metal carbonate is precipitated in the papermaking filtrate, so that carbonate adheres to the filtrate fines, which may then be more easily reused. In fact, the document teaches how to feed first lime milk and then carbon dioxide to the filtrate, where the resulting PCC is precipitated in filtrate fines. Following precipitation of PCC, either the fines containing PCC are separated from the precipitate as a thick fraction, which along with another filler is supplied for addition to the paper to be produced, or the filtrate precipitated by PCC is supplied as such to the paper production in order to be used as a filler or coating pigment.
U.S. Pat. No. 5,665,205 discusses methods aimed at improving the brightness and purity of recycled fibers used in paper production. It assumes the addition of calcium carbonate (calcium oxide or calcium hydroxide) and carbon dioxide to recycled fibers present in a consistency of 0.1-5% in a mixing reactor at a temperature of 15-80° C., the carbon dioxide and lime milk molar ratio varying between 0.1-10. By changing the reaction conditions, the size and shape of the calcium carbonate crystals may be controlled. Likewise, by changing the chemical feed order, the reaction pH may be controlled.
U.S. Pat. No. 5,679,220 discusses the process already described above (U.S. Pat. No. 5,665,205) from slightly different angles. In this document, a suspension is formed from the papermaking fiber component, whose consistency is, as above, below 5%. Lime milk is formed from calcium hydroxide, or the like, in a separate vessel. The fiber suspension and lime milk are mixed vigorously by shear forces, whereby a static mixer is considered sufficient for generation of the forces, even though pumps and other mixers may also be considered, with a gaseous reagent in a subordinate flow reactor, whereby calcium carbonate precipitates in the fibers. Either pure carbon dioxide or flue gases or carbon dioxide obtained from other similar sources are used as gaseous reagents. The document proposes that lime milk and carbon dioxide be supplied to a pipe reactor in different arrangements depending on which reactor conditions, mainly pH, are desired. Carbon dioxide may, for example, be dosed in many different steps to the reactor.
U.S. Pat. No. 5,731,080 and U.S. Pat. No. 5,824,364 especially discuss precipitation with calcium carbonate of such a fiber suspension, whose fibers contain an ample amount of microfibrils. These microfibrils are said to exist in the fibers either naturally or as the result of grinding. In the process described in the document, the fiber suspension is supplied in a consistency of 0.1-30% to a batch-type mixing reactor, receiving also lime milk. Preferably, the consistency of the suspension in the reactor is roughly 2.5%. When the reactor temperature has stabilized vigorous mixing is initiated, whereby carbon dioxide is fed to the reactor and the resulting PCC crystals precipitate on the fiber surfaces. The PCC-charged fiber suspension may be used in the paper production added in normal fiber suspension.
US-A1-20050045288 discusses bleached mechanical pulp and its production such that yellowing characteristic of mechanical pulp does not compromise the final product. The objective is optimal coating of the mechanical pulp fibers with PCC. An essential feature of the pulp described in the document is that it has been ground such that the fiber surface is well fibrillated. In the method according to the document, lime milk is added to the fiber suspension by light mechanical stirring, whereupon the consistency of the mixture is adjusted to below 10%, preferably to about 2.5%. Subsequently, gaseous carbon dioxide is added to the diluted suspension, maintaining stronger mechanical mixing than before, until all the lime milk has converted to calcium carbonate, which crystallizes in the fibers. The mixing method described above is primarily related to mixing performed on batch-type basis in a special container. According to this document, mixing may also be performed as a continuous process involving a pipe reactor containing the necessary amount of static mixers. Mixing may, for example, be carried out such that lime milk is fed to the fiber suspension flowing in the pipe, and then mixed by a static mixer with the stock. Subsequently, carbon dioxide is fed from one or more successive points to a flow, whereby static mixing occurs at each point of feeding. The stock produced in the manner described above is used as one component in paper production.
WO-A1-9942657 discusses the handling of paper machine filtrate such that the filtrate is split in two fractions. Following fractionation, lime milk is mixed into bright filtrate, and carbon dioxide into the fraction containing fines and fibers. Both these fractions are supplied to a crystallization reactor, which also receives some of the fiber components of the paper production, and whose operation may be either continuous or batch-type. In the solution disclosed in this document, the calcium carbonate going to the fraction containing the paper machine filtrate fines and fibers is converted, when treated with carbon dioxide, to calcium bicarbonate, which dissolves in the filtrate, whereupon the solids may be separated from the filtrate, if desired. Optionally, calcium bicarbonate may be precipitated back to calcium carbonate (PCC).
WO-A1-0112899 discusses the adhesion of a mineral-based filler to a cellulose suspension. The papermaking method disclosed in this document uses a cellulose fiber suspension containing alkali and/or alkali earth metal carbonates, bicarbonates or silicates. In this method, the mineral hydroxide is added to a fiber suspension, so that the mineral filler carbonate may be precipitated in the fibers. The precipitation of carbonate is based on utilizing sodium ions. The document mentions as sodium ion sources a suspension obtained in a recycled fiber process, in which sodium occurs as bicarbonate, or sodium naturally present in paper making stock, or sodium obtained from ground water. In any case, sodium ions flow in the water circulation in the papermaking machine. When, for example, calcium hydroxide is fed to a sodium-ion and bicarbonate-ion-containing fiber suspension, calcium carbonate and sodium carbonate is formed. Sodium carbonate, in turn, may react further with calcium hydroxide, thereby forming calcium carbonate and sodium hydroxide. The resulting calcium carbonate is precipitated in a known manner in the fibers of the fiber suspension. When the precipitation reaction has been completed, paper making stock is carried to the papermaking machine, where carbon dioxide is fed to the obtained precipitate in order to control the pH value, whereby sodium hydroxide obtained as a by-product in the precipitation reaction converts initially to sodium carbonate and further, when reacting with carbon dioxide and water, to sodium bicarbonate, whereupon the filtrate is ready to be recycled for paper production.
WO-A1-02066735 discusses paper production from paper fibers and calcium carbonate. To solve the problem in the document, an aqueous solution of calcium bicarbonate and/or carbon dioxide and calcium hydroxide is mixed in order to precipitate calcium carbonate as vaterite crystals, followed by immediate addition of paper fibers, whereby the vaterite crystals convert to calcite crystals adhering to the fibers. The paper making stock formed in this way may be supplied to a papermaking machine in order to produce paper.
WO-A1-03033815 refers to the document described above and discusses cellulose products consisting of cellulose fibers and PCC produced in connection with the papermaking process. According to this document, the process described there allows PCC to be deposited on any fiber surface, both inside and outside the fibers. The document describes a few different PCC precipitation alternatives.
In the first of the mentioned alternatives, calcium carbonate and carbon dioxide are mixed in a first mixer with the fiber suspension, whose consistency entering the process is 3-6%, such that carbon dioxide completely dissolves in the suspension. Subsequently, calcium hydroxide is mixed with the suspension and the suspension mixed in a second mixer, whereby the resulting calcium carbonate precipitates in the fibers.
In the second alternative, the mixture consisting of calcium carbonate and carbon dioxide is mixed in a first mixer, whereby carbon dioxide completely dissolves, and at least a part of the calcium carbonate converts to calcium bicarbonate. Finally, the mixture is fed from the first mixer to a second mixer, to which a fiber suspension and calcium hydroxide are also supplied, whereby the resulting calcium carbonate precipitates in the fibers.
In the third alternative, calcium carbonate and carbon dioxide are mixed with one another before being supplied, together with the fiber suspension, to the first mixer. This means, in practice, that a calcium bicarbonate solution is mixed with the fiber suspension. After the first mixer, calcium hydroxide is mixed in with the fiber suspension and the suspension supplied to a second mixer, in which calcium carbonate is formed, which precipitates in the fibers.
In the fourth alternative, PCC is precipitated in a short-fiber fraction and supplied to a mixing tank, in which the short-fiber fraction is mixed into the long-fiber fraction.
And, in the fifth alternative, PCC is precipitated in short-fiber pulp, and subsequently the short-fiber fraction is supplied to the multi-layer paper machine headbox, whereby, for example, a product may be produced, whose middle layer is of a short-fiber fraction and the surface layers of a short-fiber or filler-containing fraction.
WO-A1-0200999 discusses a papermaking process, in which virgin pulp is added to a flow containing at least short-fiber pulp and/or filler. This mixture is thickened and supplied to a special reactor, where calcium and carbonate ions are added to the mixture, the ions originating from, e.g., lime milk and carbon dioxide. This flow containing short-fibered material and/or a filler may be, e.g., a filtrate from a paper machine wire section. As other short-fiber sources are mentioned various chemicals and mechanical virgin pulp types of hardwood and softwood both before and after grinding, synthetic pulps, as well as recycled fibers and rejects. According to one embodiment described in this document, the precipitation of PCC in fibers takes place in a static mixer placed in a pipe flow, to which the mixture of the short-fiber/filler-containing pulp and virgin pulp described above, dilution water and lime milk is provided, such that the consistency of the suspension is in the 1-3.5% range. After the first static mixer, carbon dioxide is fed to this flow, which is mixed with the suspension in a second static mixer. The PCC-charged suspension obtained from the lime is stored in an intermediate tank, from which it is conveyed in diluted form to the papermaking machine.
EP-B1-835343 and WO-A1-03035979 discuss the production of filler-containing chemical pulp. It is based on the desired chemical pulp being produced by mixing filler, into which PCC has been precipitated, with chemical pulp. This filler is obtained by grinding the chemical or mechanical pulp such that so-called noil fibrils are produced, whose Bauer-McNett number is about P100. Calcium carbonate is mixed with this pulp containing noil fibrils, whereupon the pulp is mixed with the chemical pulp, and the mixture dried for delivery to a paper mill.
WO-A1-02097189 discusses a calcium-containing filtrate of the papermaking machine such that the calcium ions are unable to accumulate in the water circulation of paper production. This object is realized such that the pH of the filtrate is raised to at least a value of 9 without using calcium oxide or calcium hydroxide. Subsequently, carbon dioxide is mixed with the process water, which precipitates a substantial part of the calcium ions as calcium carbonate, which is removable from the process.
WO-A1-2005005726 discusses the production of paper from paper making stock, into which PCC has been precipitated dioxide in a special mixing device by utilizing lime milk and carbon dioxide.
WO-A1-2005033403 discusses a filler composition formed from a substrate and an alkali earth metal carbonate precipitated therein. The substrate may be, e.g., a paper machine filtrate containing fibers and/or fines, or even long-fiber virgin pulp. In the filler production process, solids are first removed from lime milk, after which lime milk is combined with the substrate, and PCC is precipitated from the mixture by means of carbon dioxide.
WO-A1-2005044728 describes only the production of PCC either without its precipitation in fibers or by precipitating it in a fiber flow leading to the papermaking machine. The document describes how lime milk is produced and screened free from solids, and the handling of flue gas in scrubbers and coolers, before both are carried to carbonation step at a pressure of about 1-7 bars, in which PCC is produced. If desired, pulp, in which PCC is precipitated, may also be conveyed to the carbonation step, and subsequently the pulp charged with PCC may be carried to a papermaking machine.
WO-A1-2005061386 discusses the production of PCC such that the object is to increase the crystal size and decrease the characteristic surface area. It is based on a mixture of lime milk and a suspension, which is carbonated by carbon dioxide in several successive mixing reactors in order to realize the object. The suspension may be a filtrate obtained from a papermaking machine, or another appropriate solid, or a suspension containing virgin or recycled fibers. The end product resulting from the process may be used, e.g., with other fillers and additives, as well as different fiber suspensions, in the paper production.
In other words, prior art teaches how to handle a paper machine filtrate such that PCC is precipitated in the solids therein, whether they be fiber-based or other particulate material, such as a filler, so that the solids may be more easily separated from the filtrate, or such that the solids may be utilized, e.g., as a papermaking filler.
There are also documents known in which PCC is precipitated into a fraction of the paper production, which is then conveyed either as such to a papermaking machine, or to a mixing tank for mixing with other fractions before being conveyed to the papermaking machine.
The precipitation of PCC into a fiber-containing suspension is mostly done according to the prior-art documents, such that calcium ions are arranged in the suspension, e.g., by means of lime milk, whereupon the suspension is treated by a chemical, mostly carbon dioxide, releasing or forming carbon ions. Thus, calcium carbonate crystals, which adhere to the fibers, and more specifically, to the irregularities and fibrils on their surface, are formed. Typically, this method is applied such that lime milk, and only thereafter carbon dioxide, are mixed with the fiber-containing suspension.
Another option, suggested in some documents, is the so-called bicarbonate method. In this method, carbon dioxide is mixed with liquid/filtrate/suspension containing calcium carbonate or some other suitable carbonate, whereby the carbonate is converted to bicarbonate. When calcium or some other hydroxide suited for the purpose, e.g., lime milk, is added to the solution/suspension, carbonate is formed in the reaction of hydroxide and bicarbonate.
However, both the conventional and bicarbonate methods have their own weaknesses. The bicarbonate method requires the use of sodium in at least some form, which by no means is always present in the paper making stock without separate addition. It is our understanding that in practice the same problems appear in to the bicarbonate method as in the conventional method discussed below. The conventional method, in which lime milk, and only thereafter carbon dioxide, are mixed first with an applicable liquid or suspension, is not able, in our opinion, to control the formation of carbonate crystals in an as optimal way as is required in the production of more demanding products. Moreover, the rate of the precipitation process has not been necessarily sufficient, when attempting to perform the precipitation of PCC directly in the paper machine short circulation without a separate, longer-lasting side circulation.
In our opinion, the slow rate of the precipitation reactions in the present processes is caused by the way in which the chemicals are mixed. The prior-art solutions attempting to perform precipitation of PCC in a medium flowing in a pipe disclose that both a mere pipe flow, and either static or dynamic mixers arranged in a pipe, are sufficient for causing an adequately good mixing. However, the end result is that production of PCC by these methods is not such that it would be ready for general marketing or use in paper mills.
For example, in the prior-art document U.S. Pat. No. 5,679,220 referred to above, it says that carbon dioxide is allowed to discharge into a pipe mixer such that the fiber suspension flowing in the pipe sweeps up the carbon dioxide entering the pipe, and the pipe flow in itself mixes the carbon dioxide as small gas bubbles with the stock. In the test equipment of this US document, in which the diameter of the flow pipe is between half an inch and six inches, a reaction time of about one to two minutes is needed, which is altogether too long in terms of the paper machine short circulation, in which the flow velocity of the stock in the pipe is roughly 3-6 m/s. In the mixing method according to this US document, the following occurs in practice, when carbon dioxide is swept along with the flow. After the point of adding carbon dioxide, a uniform gas wake initially forms, which slowly breaks into gas bubbles that gradually burst further into small bubbles. However, long after feeding the carbon dioxide, a situation prevails in which only a small part of the suspension liquid is able to interact immediately with the carbon dioxide bubbles. This part of the liquid is very quickly saturated by carbon dioxide resulting in slow dissolution of carbon dioxide, since the bubbles must be carried further in the suspension in order to encounter liquid not yet saturated by carbon dioxide.
As already briefly mentioned the slow mixing described above leads to uneven size distribution of the PCC crystals, since during the whole mixing period and until at least one of the chemicals has been fully consumed, existing PCC crystals grow and new crystals form. Moreover, in suitable flow conditions, PCC also precipitates on the walls of the flow channel, or the like, or on other fixed structures. This naturally is aided by the fact that the crystallization reaction is protracted, whereby far-away structures susceptible to crystallization become available.
In practice, these problems can be solved by trying to arrange a sufficiently short chemical mixing time, especially reaction time. In practice, this means, for instance, that when an optimally evenly dispersed PCC is desired, carbon dioxide is dissolved in the liquid or suspension flow, and only when the time needed for almost complete dissolution of carbon dioxide has expired, is lime milk mixed with the flow and fed in at least an almost stoichiometric amount relative to carbon dioxide. And, as in this case, lime milk is mixed as quickly and evenly as possible, the crystallization reaction gets to start and advance evenly through the whole liquid, whereby crystal growth is even and the reaction progresses quickly until completion.
French patent application FR-A1-2 821 094 describes an attempt to precipitate PCC in stock as vaterite crystals and focuses on solving the problem caused by unstable vaterite. In other words, it is known that vaterite is the most unstable crystal form of calcium carbonate, which tends to convert quickly to calcite and aragonite. Since the applicant of the document believes that the presence of vaterite in the final product provides the end product with especially good properties, a method was developed in which vaterite is formed at a sufficiently late stage for it to be sustained until the end product. The only way to ensure that the production of crystals occurs sufficiently late is to feed another source material for calcium carbonate at such a late stage that carbonate crystals form at the desired stage. In this case, the applicant decided to feed lime milk as close to the paper machine headbox as possible. The document uses the term “immediately before web formation” in many different contexts for when lime milk is fed. The penultimate sentence of the specification in the document states that lime milk is fed less than ten seconds before the point, when stock enters the web formation area of the papermaking machine. In other words, the carbonation reaction proceeds toward the end within ten seconds and the stock travels through the headbox to the paper machine wire section. The applicant therefore seems to believe that the precipitation or crystallization reactions may proceed without a problem in the piping of the paper machine headbox.
Studies that were made, however, have shown that such late lime milk feeding, using mixers for feeding lime milk available at the time of the patent document, in all probability is the cause of major runnability problems in a papermaking machine. The most obvious and serious of these appear as precipitation in the headbox pipes, which instantly harms web formation. The main reason for these runnability problems and precipitations is that for the most part, lime milk is mixed only with the stock due to the turbulence effect existing in the flow, with the result that the carbonation reaction continues at least in the papermaking machine headbox, and possibly also in the paper machine wire section. Thus, it should first be noted that the idea of using PCC as vaterite crystals, regardless of its good intention, will fail if it means that the crystallization reaction must proceed in the pipes of the paper machine headbox. Second, due to the great instability of the vaterite crystals, they cannot be produced in advance, because of their rapid conversion to more stable crystal forms.
Identifying the many problems related to the production of PCC discussed above requires a closer look at the kinetics of the crystallization reaction. If it is assumed that carbon dioxide is fed and at least the majority thereof also dissolved in the liquid or suspension/paper making stock, in which PCC is preferably to be precipitated, then the PCC crystallization or precipitation reaction will start at the time of mixing the lime milk. The time needed for the relevant mass transfer is influenced by, e.g., the following two factors: The faster and more efficiently lime milk is thoroughly mixed with the flow, the shorter the reaction time will be. Fast and efficient mixing in this step is intended to reduce as much as possible the differences of concentration in the flow. Another important factor is the lime-milk particle size, i.e., the smaller the lime-milk particle size is, the faster is the mass transfer of the relevant particles into the liquid phase, whereby the rate of the crystallization reaction obviously also increases. When the PCC starting materials are fine-grained and evenly dispersed in the liquid flow, crystallization occurs quickly and evenly through the whole liquid flow, eliminating the possibility of forming of oversized PCC crystals, agglomerates and precipitates. Experiments that were made have shown that a suitable lime-milk mixing time in terms of size distribution for producing PCC used in conventional paper production is roughly below 3 seconds. Furthermore, when the feed, mixing and reaction zone are arranged such that the PCC precipitation reaction ceases upon substantially completed conversion before the desired process step, e.g., the paper machine headbox, it is ensured that harmful precipitation or runnability problems will not occur in the relevant process step and the subsequent process.
The process according to the invention may also be implemented such that first lime milk, and only thereafter carbon dioxide, is mixed with a liquid or suspension (taking into account the influences of the pH value on darkening of the stock). Experiments that were made have shown that in a precipitation reaction carried out according to the method of the invention, in which carbon dioxide and lime milk were fed in whichever order, when trying to obtain optimal PCC crystal-size distribution, the time needed for the whole reaction, from the start of feeding the component participating in the reaction until practically all the lime milk has reacted with carbon dioxide and PCC crystals have formed, in other words, substantially a 100 percent conversion has taken place before a desired process step, e.g., the headbox of the fiber web machine, is below fifteen seconds, preferably below ten seconds, more preferably below 6 seconds, and most preferably below 3 seconds. The shorter the reaction time sought for, the quicker the mass transfer needs to be. Quick mass transfer is only obtained by arranging the feed of the latter chemical such that in practice the chemical is mixed almost completely, immediately when the feed occurs. It is, naturally, assumed that the previously fed chemical has already been dispersed, or even dissolved evenly through the whole liquid or suspension/paper making stock. Another important factor in aiming for a short conversion time is, as previously mentioned, a sufficiently fine bubble or particle size, i.e. a large specific surface area of the chemicals in the reaction zone. The smaller the bubble or particle size (the greater the specific surface area) is, the quicker the mass transfer from the gas or solid material to the liquid occurs. The preferred average particle size of the relevant lime milk was found to be below 3 microns (μm), preferably below 1.5 microns, and most preferably below 0.5 microns. As for the bubble size of carbon dioxide, it should be at least below 10 mm, and more preferably below 100 microns. Naturally, the optimal result is obtained if carbon dioxide, when fed along with the liquid flow, is already dissolved completely in the feed/injection liquid.
In theory and under certain preconditions, useful mixing methods comprise static mixers, dynamic (rotating) mixers and injection mixers. Static mixers are suited for situations, in which all conditions are optimal. In other words, the liquid flows to be mixed are astatic and their flow velocities are not too far apart from each other. Moreover, the mixer has to be specifically designed for the purpose of rapid mixing. A particular problem may be the flow resistance caused by large-sized mixers, the high cost of producing a mixer, or the suitability of mixers for very few applications given their costs of development.
Dynamic mixers are also possible, especially when relatively small flows should be mixed with one another. Relative differences in the flow velocities of liquids to be mixed can be large, when the mixing volume is kept relative small. In large-sized devices, which often comprise, e.g., the piping of a papermaking machine headbox, the investment and installation costs of a rotating mixer and the consumption of energy are factors restricting their use.
Based on tests that were made, the most advantageous mixing method related to the application purpose of the invention turned out to be an injection mixer.
TrumpJet®, a feed device known from prior art and developed by Wetend Technologies Oy, is discussed in patents such as EP-B1-1064427, EP-B1-1219344, FI-B-111868, FI-B-115148 and FI-B-116473. The feed device was developed, due to the fact that only rotating and static mixers for mixing retention agents and similar chemicals in the production of paper were known in prior art. The devices were expensive investments in every aspect and relatively ineffective in situations, where even mixing of the chemical with the headbox stock is expected, which prompted the development of, e.g., an injection mixer according to the invention allowing for even and quick mixing of chemicals in a highly user-friendly device. Thus far, the functional quality of such a device has been measured by the resulting product, i.e., in most cases paper, as well as the amount of chemicals used. In other words, when the resulting paper quality remained unchanged or improved while the amount of chemical utilized declined, it has been inferred that chemical mixing had improved when compared with the use of prior-art devices.
However, the injection mixer is now assumed to implement a new kind of application purpose, where we are dealing with not only mixing a single chemical with the paper making stock such that it disperses evenly therein, when carried through the papermaking machine headbox to the wire, but with mixing two chemicals with the paper making stock, such that the reaction among the chemicals occurs upstream of the headbox or some other process step.
Thus, an object of the present invention is to arrange such a short mixing time for the chemical and the mixing itself so even throughout the whole liquid flow that the size distribution of the resulting PCC crystals is as homogeneous as possible and the risk of forming of oversized crystals, agglomerates and precipitates as little as possible.
A more specific object of the present invention is the injection of lime milk and/or carbon dioxide with the liquid or suspension flow such that the mixing into the flow occurs quickly and evenly, in practice independently the own turbulence of the flow.
When injecting carbon dioxide and/or lime milk into the flow, preferably the TrumpJet® feed device developed by Wetend Technologies Oy and already described above is used, so that a desired number of them may be placed on the flow pipe circumference. When one or more feed or injection devices are placed on the circumference of the flow pipe, depending on the size and shape of the pipe, we are actually dealing with an injection unit covering all the injection devices placed on the same circumference of the pipe in order to feed the same chemical. FIGS. 19, 20 and 21 show results of the test, in which the operation of a feed device according to the prior art and the TrumpJet® feed unit is compared, when a chemical is fed into the liquid flow. The feed device according to the prior art is a pipe fitting arranged on the flow pipe circumference, from which a chemical to be mixed is allowed to flow along with the liquid flowing in the pipe. It can be seen from the figures that the TrumpJet® feed unit is capable of the quick mixing set as one condition of the invention above. By using the TrumpJet® feed device, injection and mixing may be performed at a distance of about 1-3, preferably about 1.7-2 seconds (calculated in meters, less than ten meters) from the subsequent process step or device, while when using a prior art device the feeding the additive is to be done several tens of meters before the desired process step or device, e.g., the papermaking machine headbox, so that the chemical has time to be mixed under the influence of the flow turbulence before the process device. The above applies to situations, in which the chemical or the like should be evenly distributed before the following process step. If, however, we are dealing with a situation, in which the chemical must react with some other chemical or a substance already present in the liquid flow, the time required by the respective reaction must naturally be reserved for, unless it is preferred to allow the respective reaction to continue in the above-mentioned process step, e.g., the papermaking machine headbox.
Another object of the present invention is to control the PCC crystallization in the flow solid, whereby both the size and distribution of the carbonation crystals in the suspension may be determined beforehand in a relatively accurate fashion. An essential way of realizing this object consists in injecting the PCC starting materials in the liquid flow such that their crystal and/or bubble size is suitable for the purpose.
An object of the present invention is to precipitate PCC crystals on the surface of the fibers in the flow.
A special object of the present invention is to precipitate PCC crystals in the hollow core, the so-called lumen of the fibers present in the flow.
In order to realize at least one of the above-mentioned objects and to accelerate the PCC precipitation reaction the present invention discloses the addition of at least one of the chemicals: lime milk and carbon dioxide, in a bubble or particle size sufficiently fine, in a fluid flowing in a pipe such that, in practice, simultaneously with the addition of a chemical, it becomes substantially evenly mixed over the whole cross-section area of the flow. Here, the injection should occur substantially in transversal direction relative to the liquid flow, and with a velocity at least three times (preferably 5-10 times) that of the liquid flow.
In our opinion, this type of mixing is especially important for carbon dioxide particularly when it is the latter of chemicals to be mixed, which the prior-art methods have not been capable of mixing in the liquid or the fiber suspension so well that the carbon dioxide would have dissolved quickly in the water, in practice immediately, which is a requirement for in-line PCC production to succeed on a paper-mill scale. When carbon dioxide already in its addition step is distributed over the whole cross-section area of the flow, and carbon dioxide is not able to form at any point of the flow densification, in which the liquid surrounding the bubble would be saturated by carbon dioxide and the dissolution of carbon dioxide prevented, but instead carbon dioxide becomes dissolved immediately after injection.
When, according to an especially advantageous embodiment of the invention, also lime milk is fed and mixed over the whole cross-section area of the pipe flow, a situation ensues, in which the lime milk mass transfer occurs quickly, whereby calcium ions are in fact evenly distributed through the whole liquid/suspension. Thus, their reaction with the carbonate ions resulting from the dissolving of carbon dioxide is actually able to start simultaneously through the whole liquid volume, in which also the originally present solid may also be assumed to be evenly distributed. In other words, the fiber flow will be treated homogenously and evenly. The result is that the crystal size and calcium carbonate distribution remain even and crystals are situated evenly in the suspension solids. If mixing is uneven and lime milk reacts strongly locally, uncontrolled PCC crystal growth occurs, which may lead to oversized crystals and PCC agglomeration, among other things, and cause serious quality and process runnability disturbances. This may also cause PCC to crystallize uncontrollably on the walls of the process devices and piping causing overwhelming cleaning and process runnability problems. Similarly, process controllability and adjustment is affected, and predicting the quality of the produced PCC becomes more difficult.
At least some of the weaknesses of the PCC production process according to the above described prior art may be remedied and at least some of the above-mentioned objects may be realized by a method according to the invention for crystallizing a filler, especially calcium carbonate, in a paper machine short circulation so as to form crystals in a flowing solid and/or on its surfaces by feeding carbon dioxide and lime milk into a short circulation liquid flow and allowing them to react with one another, whereby the method is characterized in that at least either carbon dioxide or lime milk is fed and mixed into the liquid flow as sufficiently small-sized particles or bubbles such that the chemical is spread substantially evenly in the liquid flow, regardless of the flow conditions of the liquid flow, and such that the crystallization reaction is substantially completed in less than fifteen seconds, preferably less than ten, more preferably less than six, and most preferably less than three seconds, thereby realizing a size distribution of the homogenous calcium carbonate crystals suited for the purpose, preventing the formation of oversized PCC crystals, PCC agglomeration and PCC precipitation, and controlling the carbon dioxide and lime milk carbonation reaction.
In quite the same way, at least some of the weaknesses of the above-described PCC production process according to the prior art may be remedied, and at least some of the above-mentioned objects may be realized by an approach system of a fiber web machine according to the invention, comprising at least devices for receiving filtrate from the fiber web machine, devices for producing paper making stock at least from the filtrate obtained from the fiber web machine and various fiber and filler components, as well as flow piping, along with pumping device(s), consisting of a plurality of flow pipes for carrying paper making stock from its production to the headbox of the fiber web machine, which approach system is provided with devices for feeding both carbon dioxide and lime milk into a liquid flow moving inside flow piping, whereby the approach system is characterized in being provided with a first injection unit arranged on the wall of the flow pipe for injection of at least either lime milk or carbon dioxide into the flow pipe, substantially transversally relative to flow direction of the liquid flow.
Other characteristic features of the method according to the invention and the approach system of the fiber web machine appear in the attached claims.
Advantages obtained by the method and the approach system of the fiber web machine according to the invention over the prior-art methods are for example:                PCC precipitation directly into the suspension flowing in the short circulation of the fiber web machine        Rapid dissolution of carbon dioxide so that the PCC production process may be connected immediately in the paper machine short circulation        Quick mixing of lime milk in the suspension, quick mass transfer from solid to liquid phase        Rapid carbon dioxide and lime milk reactions        Even mixing of the chemicals through the whole flow meaning even and controlled formation of carbonate crystals and even adherence of crystals to the fibers and solid        A simplified short-circulation process—no need at all for a thick stock mixing tank or restriction to use a smaller-sized tank        Reduction of the investments required by PCC production by at least half compared with previously used devices        Reduction of the PCC production energy costs to about a tenth when compared with the previous PCC production        The consumption of clean water is substantially reduced, when compared with on-site PCC devices according to prior art        Paper production requires less or no retention chemicals        Paper production may use more fillers than previously, thereby saving on the use of expensive fiber material        The need for hydrophobic glue is reduced        The papermaking water cycles become cleaner and/or the need for purification chemicals is reduced, and water cycles may be closed more than previously        